Manufacture of methylolalkanes is carried out in a variety of processes including by the reaction of formaldehyde with another aldehyde with formaldehyde (hereinafter sometimes referred to as reactant aldehyde), that is, an aldehyde having at least one hydrogen bound at the α-carbon atom adjacent to the carbonyl moiety. The base-catalyzed aldol reaction of the reactant aldehyde with formaldehyde initially generates the methylol derivative of the aldehyde in the first reaction step. Then the aldehyde moiety may be converted in a second reaction step by reaction with further formaldehyde and base in a Cannizzaro-reaction into an alcohol group. Simultaneously, the formate of the base is generated. The 1st reaction step, the aldol reaction, and the 2nd reaction step, a Cannizzaro reaction, may either be carried out separately or in one working step. The bases used both for the base catalyzed reaction step 1 and also for the reaction step 2 which is stoichiometric in relation to the base quantity may optionally each independently be, for example, alkali metal or alkaline earth metal hydroxides, carbonates or tertiary amines. In the so-called inorganic Cannizzaro process, an inorganic base is used, such as sodium hydroxide, potassium hydroxide or calcium hydroxide. The resultant formates, such as potassium formate or calcium formate can be used in further industrial applications such as an assistant in the leather industry.
The reactions of formaldehyde with acetaldehyde, propionaldehyde, n-butyraldehyde and isobutyraldehyde are of particular interest. The corresponding reaction products are pentaerythritol, trimethylolethane, trimethylolpropane and neopentylglycol. These are polyhydric alcohols of great industrial significance which find use, for example, in the field of coating resins, power coating, foam production and polyester production.
In particular, the manufacture of TMP according to the inorganic Cannizzarro process is disclosed, for example, in U.S. Pat. No. 3,183,274, U.S. Pat. No. 5,948,943, U.S. Pat. No. 7,253,326 and U.S. Pat. No. 8,354,561. Batchwise production of TMP is seen in U.S. Pat. No. 7,253,326 to Eom et al., wherein the batch production is followed by a semi-continuous product recovery train. While batchwise production may be advantageous in terms of raw material use, such systems are relatively difficult to operate and capital costs are higher than continuous systems.
TMP is prepared from n-butyraldehyde and formaldehyde. In one preferred process, base-catalyzed aldol reaction initially generates 2,2-dimethylolbutyraldehyde in a first reaction step which is then converted to a TMP-formate mixture by way of a Cannizzaro reaction. The TMP-containing mixture is typically extracted with an organic solvent, such as ethyl acetate, thereby providing an organic phase comprising TMP and an aqueous phase containing the formate. The solvent is separated and the crude TMP is purified by distillation. Typical processing is seen in U.S. Pat. No. 5,603,835 to Cheung et al., Comparative Example 1, Col. 7. See, also, U.S. Pat. No. 5,948,943 to Supplee et al. referred to above.
The reaction of the aldehyde with formaldehyde is highly exothermic and can result in excessively high temperatures in the reaction zone before the heat can be removed. The temperature spikes lead to efficiency losses due to side reactions. In order to reduce said temperature spikes, the art generally teaches to use a relatively dilute aqueous formaldehyde solution and aqueous solution of the inorganic base in order to moderate temperature. Because of the presence of large amounts of water in the reaction mixture, the heat capacity is relatively high so that the exothermic heat of the reaction does not raise the temperature of the mixture to a level above the desired range.
Besides the large amount of water, it is conventionally typical to use formaldehyde in substantial excess over the theoretical amount based on the reactant aldehyde. In cases where n-butyraldehyde is reacted with formaldehyde to produce trimethylolpropane the art teaches generally a formaldehyde excess of about 1 to 7 moles or so over the formaldehyde needed for the actual reaction.
Commonly, the aqueous formaldehyde solution is blended with the starting aldehyde continuously to produce a stream of aqueous mixed aldehydes and the aqueous solution of the inorganic base is injected into this stream in a mixing zone. The reaction mixture is then fed to a reaction zone. Heat generation is most problematical at or near the mixing zone where the reactants are most highly concentrated. Heat generated in these areas leads to temperature spikes and byproduct generation. As will be appreciated from the foregoing references, byproducts can cause color and other product quality problems, leading to higher purification expense in addition to loss of efficiency because of lower yields. Moreover, large amounts of water needed as a temperature moderator are difficult and expensive to process.